A piping system of a steam turbine facility includes a steam piping system that guides steam generated by a boiler to a steam turbine. The steam piping system includes drain piping that discharges drains produced from cold piping and equipment when the steam turbine is warmed-up for operation.
Among the steam piping system, the steam piping connected to a first-stage, high-pressure turbine is referred to as main steam piping. Drain piping for warming-up is connected to various devices, e.g., a steam regulating valve for regulating a steam flow rate, connected to the main steam piping on the downstream thereof, and also to an inner casing surrounding a steam turbine main unit rotated by a steam power disposed further downstream.
A shut-off valve for adjusting the steam flow rate or shutting off the steam is interposed in each drain piping. The drain produced during the warming-up is guided and drained into, for example, a condenser whose internal pressure is lower than that in the main steam piping and that of equipment. The shut-off valve is closed after the warming-up is completed.
In a typical steam turbine equipment, an upper half-side main steam piping and a lower half-side main steam piping are connected to the turbine so that steam can be introduced into the turbine from both the upper half side and the lower half side thereof. In general, each main steam piping is provided with a main steam regulating valve having drain piping.
In a steam turbine piping system as that described above, steam pressure fluctuations occur in a pipeline downstream of the main steam regulating valve, for example, at a portion in the main steam piping downstream of the main steam regulating valve, or at a portion in the drain piping downstream of a valve seat of the main steam regulating valve. Probably due to the design layout of the main steam piping, the steam pressure fluctuations in the main steam piping downstream of the main steam regulating valve are greater in the upper half-side main steam piping and smaller in the lower half-side main steam piping.
Pressure fluctuations occur also inside the inner casing that surrounds the rotating steam turbine due to, for example, current transformation of steam that flows in and the rotating turbine. This results in pressure fluctuations in the drain piping connecting to the inner casing.
A phenomenon is observed in which, in the drain piping connected to the main steam piping or the turbine casing, the temperature of the portion of the drain piping upstream of a shut-off valve abnormally increases during the load increase from the starting-up to the rated operation, if the shut-off valve is closed. Such an abnormal increase of temperature causes material strength of the drain piping to be reduced. When stress produced in the piping by the piping pressure exceeds a permissible level of the piping material under the corresponding temperature, a failure such as burst might occur.